The exemplary embodiment disclosed herein relates to document processing systems. It finds particular application in conjunction with sensing and control of banding and will be described with particular reference thereto. However, it is to be appreciated that the exemplary embodiment is also amenable to other like applications.
By way of background, in a typical printing system, a photoconductive drum or photoreceptor (PR) rotates at an angular velocity. See, for example, U.S. Publication No. 2006/0251437, entitled “TONE REPRODUCTION CURVE SYSTEMS AND METHODS,” which is incorporated by reference herein. As the photoconductive drum rotates, the photoconductive drum is electrostatically charged. A latent image is exposed line by line onto the photoconductive drum using a scanning laser, for example, using a rotating polygon mirror. The latent image is developed by electrostatically adhering toner particles to the photoconductive drum. The developed image is transferred from the photoconductive drum to the output media such as paper. The toner image on the paper is fused to the paper to make the image on the paper permanent. The surface of the photoconductive drum is cleaned to remove any residual toner on the surface of the photoconductive drum.
Typically, the printing device drives the photoconductive drum using a motor drive system or a motor drive train. The motor drive system has a substantial amount of external loading because it typically drives the auxiliary rollers and transports the paper through a series of gear trains. With the additional external loading, as well as periodic disturbances due to imperfections in the series of gear trains, the motor drive system imparts a varying velocity on the photoconductive drum.
The varying photoconductive drum velocity causes scan line spacing variation in the printed image. The scan line spacing variation is a significant contributor of artifacts in the marking process. For example, halftone banding caused by scan line spacing variation is one of the most visible and undesirable artifacts, appearing as light and dark streaks across a printed page perpendicular to the process direction.
Banding is thus defined as a one dimensional image density variation in the process direction. It is often periodic and it can result from errors in the mechanical motion of rotating components within a marking engine. These components may be gears, pinions, and rollers in the charging and development subsystems, photoreceptors and their drive trains, or the ROS polygon. Several methods have been proposed to perform feedback compensation of banding using image based controls techniques. Such methods involve measuring the banding induced density variation using an imaging system such as an offline scanner or an in situ full width array sensor, or a point sensor such as an ETAC. Based on the density variation, a controller calculates a periodic compensation signal that is injected into the system, either into the imager (ROS), into a power supply to effect a bias, or into the image itself. These methods require an accurate profile of the density variation. Since the density variation is periodic, it can be characterized by the frequency, amplitude, and phase of its fundamental, as well as its harmonics. The frequency of the banding defect can be measured using Fourier analysis, but is typically known ahead of time based on the mechanical design of the marking system.
Of the three banding characteristics, frequency, amplitude, and phase, banding phase is probably the most difficult characteristic to measure. However, for feedback compensation, it may be the most important characteristic. Banding phase is difficult to measure since, relative to the printed page, it varies from page to page. That is, the banding may have a density peak at the beginning of page one, but may have a density trough at the beginning of page two. Thus, if one measured page one, and determined that the phase was such that a density peak occurs at the beginning of each page, upon applying a corresponding compensation to page two, poor performance would result—the banding may even be enhanced by the control system, rather than suppressed. The problem is that the banding source is not synchronous with the print. Thus, one cannot use the print as a phase reference.
Actuation problems for banding compensation also occur. For example, in using the image itself for banding compensation, phase synchronization is difficult as the page is usually not synchronous with the banding source. Further, use of the image has limitations in solid areas in terms of amplitude quantization and in the potentially high cost of image computations. Use of exposure modulation for banding compensation can also have limitations in terms of exposure bandwidth, noise rejection, and potentially high cost exposure system modifications. The photoreceptor servo control system attempts to minimize the effect of banding defects. However, the shaft encoder typical of these systems does not measure the photoreceptor surface velocity, which is the key parameter for banding print defects. Further, encoders can have run-out and have their own inherent error sources.
Thus, there is a need for a method and system that compensates for the periodic banding defects by measuring the output density variation in interdocument zones, customer image zones, or on printed pages and then modulating the rotational speed of the photoreceptor shaft using its motor.